The present invention relates generally to current sensing devices for electrical systems, and more particularly to automatically resettable alternating current fault indicators.
Various types of fault indicators have been constructed for detecting electrical faults in power distribution systems, including clamp-on type fault indicators, which clamp directly over cables in the systems, and test point type fault indicators, which are mounted over test points in cables or associated connectors of the systems. Such fault indicators may be either of the manually reset type, wherein it is necessary that the indicators be physically reset, or of the self-resetting type, wherein the indicators are reset upon restoration of line current. Examples of such fault indicators are found in products manufactured by E. O. Schweitzer Manufacturing Company of Mundelein, Ill., in U.S. Pat. No. 3,676,740, 3,906,477, 4,063,171 and 4,234,847 of the present inventor, and in the copending application of the present inventor; Fault Indicator with Magnetic Test Point, Ser. No. 103,042, filed Dec. 13, 1979.
Detection of fault currents in self-resetting fault indciators may be accomplished by means of magnetic switch means such as a magnetic reed switch in close proximity to the conductor being monitored. Upon occurrence of an abnormally high fault-associated magnetic field around the conductor, the switch means actuate a trip circuit which produces current flow in a trip winding to position a target indicator visible from the exterior of the indicator to a trip or fault indicating position. Upon restoration of current in the conductor, a reset circuit is actuated to produce current flow in a reset winding to reposition the target indicator to a reset or non-fault indicating position.
The present invention provides an improvement in such fault indicators, in that it provides a simplified circuit for use in conjunction with a single bidirectional magnetic winding in place of separate trip and reset windings. A prior art circuit which purported to provide such a single bidirectional magnetic winding is shown in U.S. Pat. No. 3,720,872 to Russell et al. However, this circuit provides fault indications on only one-half of the alternating current cycle, and therefore is not practical for many applications. In contrast, the invention provides fault indications on both halves of the alternating current cycle, and does not rely on reversal of line current to provide a reset current for its indicating mechanism.
The invention, because of its simplicity and compactness, is particularly attractive for use in remotely indicating fault indicators, such as are utilized in underground electrical distribution systems. In such systems, wherein primary and secondary feeder cables are directly buried in the ground and are brought to the surface only for connection to pad mounted distribution transformers or other system components, the need exists for economical remotely-indicating fault indicators which monitor both halves of the alternating current cycle and are suitable for installation within the above-ground metal enclosures typically utilized to house and protect the system components. Preferably, such fault indicators include a circuit module in magnetic communication with the monitored conductor, and an indicator module from which circuit status can be determined from the exterior of the enclosure without compromising the security provided to the power system components by the enclosure against vandalism or theft.
One successful remotely-indicating fault indicator, wherein circuit status is indicated by the magnetic sense of a remote magnetic test point module, is described in the previously identified copending application of the present application, Ser. No. 103,042. The present invention provides an improved circuit for this fault indicator, and for other remotely indicating fault indicators wherein a magnetic winding is utilized to selectively magnetize an indicating element, which reduces the size of the indicator module, and reduces the number of conductors required between the modules.
Accordingly, it is a general object of the present invention to provide a new and improved fault indicator.
It is a more specific object of the present invention to provide a new and improved remotely-indicating fault-indicator which is more compact and economical to produce.